Technical Field
Embodiments of the invention relate generally to power converters. Particular embodiments relate to dual active bridge power converters.
Discussion of Art
Power supplies are electronic/electrical circuits that supply electric power to one or more electric loads. The term “power supply” is most commonly applied to collections or an assembly of electrical devices that convert one form of electrical energy to another and are commonly referred to as “power converters.” Many power supplies include two or more power converters connected together. Typically, power converters are “switching” power converters, in which multiple solid state devices are used to rapidly and intermittently interrupt or commutate an input current so as to effectuate conversion of the input current to an output current having different amplitude, voltage, and/or frequency. For example, a “DC power converter” produces output power at a substantially constant output voltage and/or current.
Conventional power converters, generally, are groupings of plural solid state switches that are connected to output terminals from a first DC input terminal or from a second DC input terminal. The two DC terminals typically are known jointly as a “DC link,” while the term “DC link voltage” often is used to refer to a potential difference across this DC link. Power conversion typically is a dynamic process that requires rapidly sequenced changes in state of the solid state switches. Although the switches exhibit high conduction or resistance in their closed or open steady states, in transition between states the solid state switches typically exhibit capacitive and resistive “switching losses.”
Power converters can be designed on a dual active bridge (DAB) topology to provide an adjustable bi-directional power flow between two isolated DC links over a broad range of voltage ratio. Conventionally, power flow control is achieved by adjusting only the phase shift between the primary and the secondary side. This control is simple to implement and allows a large operating range. However, phase shift control can drive large currents inside the converter components when the output voltage ratio is substantially different from the transformer ratio. The result in higher conduction and switching losses of the semiconductors and the current may also exceed the maximum current capability of the devices. Hence, the DAB may not be operable at certain voltage ratios even at reduced power levels. Accordingly, conventional DABs typically are operated only in a relatively small voltage ratio range for which they have been designed. In the literatures, special operating modes are proposed that allow improved operation in some special operating points. Switching between modes transiently can have at least the following adverse effects: discontinuities of the switching pattern (leading to unpredictable and possibly excessive electrical transients within the switches), gaps in the operating range, different transfer gains resulting in control instabilities.